Gear Ratio Question

black06pony · Feb 6, 2011

I'm a newbie to engines and stuff but I'm trying to learn. My GT has a 3:31 Rear Axle Ratio. I have seen options for a 3:55 and 3:73. Several shops have said I could put a 3:73 Gear ratio in it and it would run faster. My 1st question is, what do these numbers mean? My 2nd question is, would a 3:73 work in a 06 GT Convertible? I have been on Ford's website and configured a couple of new GTs. The convertibles are always ONLY available with a 3:3I, even with a Shelby.

SpartaPerformance · Feb 6, 2011

The gear ratio is how many revolutions the rear wheel have to the drive shaft, so 3.31 turns compared to 3.73 turns. The vehicle also accelerates quicker because torque to wheels is multiplied by a larger number. For example

300lb ft of torque * 3.31 = 993
300lb-ft of torque * 3.73 = 1119

mbuckcoyote · Feb 6, 2011

While that's an interesting take on gear ratios, the more common explaination is: how many teeth on the pinion gear there are as opposed to the ring gear. for example: 3.73= 11 teeth on pinion to 41 teeth on ring. 3.31 would be something like 10 teeth on the pinion to 33 on the ring. That's not completely correct but, close on this last one. The higher the number on the ratio (3.73, 4.11, etc.) the quicker the acceleration. But, will cost in top speed. The inverse is true for lower gears.

black06pony · Feb 6, 2011

Thanks! I think I understand. I just want to feel a quicker jump when I punch it. I'm not racing or anything, so top speed probably doesn't matter as much for me. I think I may have reached 100mph in this thing 1 time before I backed off of it.

j_lutter31 · Feb 6, 2011

Another viewpoint.....

Say you have your 3.31 gears in your car. Maybe you have thought about upgrading to 3.73's or 4.10's. Well the higher you go with your ratio, the higher your rpm will be at any given speed.

For example, now at 70 mph you might be turning 2000 rpm, with the 4.10's it might be closer to 2400. Which will slightly hurt your fuel economy on the highway. Something to think about, some people will say it doesn't affect it enough to matter, some say otherwise....

demea · Feb 7, 2011

I think we should know if you are 5sp or auto?
4.10's for auto and 3.73's for 5sp is generally what people are running unless you plan on going SC, then the general consensus is to stay @ 3.73 for an auto.

My auto 06 vert GT has Ford Racing 4.10's and is 2500 @ 70mph fyi.
Gas mileage is down by about 15-20 miles per tank, but the fun factor is up 1000%
Especially with a custom tune and CAI, that all you really need in these cars.

black06pony · Feb 8, 2011

I have an automatic but it's convertible. I don't know why a convertible should matter, but I've noticed that Ford doesn't put anything except a 3.31 in the convertibles. You can't even order them with anything else. Looks like they do it that way for the Shelby convertible too. I can't figure out why.

ct07gt · Feb 9, 2011

They probably assume that performance is a lower priority to people buying convertibles. They might have been trying to keep the CAFE number up a little by keeping the convertibles' mileage up with the 3.31. There is no difference in the rear end between a coupe and convertible, both use the 8.8" rear. Only the V6 had the weaker rear end. You can use any ratio made for the GT.

marktexaf · Feb 10, 2011

Related Question

I would like to piggyback on this question. I bought my '08 GT used from a dealer and there were some mods done to it which I'm still trying to figure out. One that I suspect is the gears. Currently when I'm going 60 mph per the speedo, my GPS (two different ones) showed that I was going 70 mph. During a visit for scheduled MX, I asked the dealer to check on it. They said the wheels/tires are the original diameter. They said to find out for sure, they would have to take the diff apart. Question, with this info, do you think the gears were changed and what ratio do I have now? Looks like there is about a 10% difference with the speedo registering slower than the actual speed.

ct07gt · Feb 10, 2011

That would be an unusual change to a taller ratio, I have not heard of anyone making that switch. We would have to know the tire size and RPM at a given speed to get the ratio for you.

marktexaf · Feb 12, 2011

Numbers

Here's the numbers on two separate runs..

Tire size= 235 x 50 x 18 (Ht 27.25)

Run 1:
RPM 2K
Speedo 61 Mph
GPS 52 Mph

Run 2:
RPM 2.3K
Speedo 75 Mph
GPS 65 Mph

ct07gt · Feb 14, 2011

Assuming you have a manual transmission and the speedometer is correct, it sounds like you have a 3.73 according to this calculator. JawsGear.com

You have a .68 transmission ratio in 5th. If the GPS is correct, then you might have a 4.10 rear end.

lokstag1 · Mar 26, 2011

put car in neutral, jack up rear end. have a buddy hold one wheel. slowly rotate the other while paying attention to the drive shaft. Count the revolutions of your drive shaft to determine the gear ratio in your pumpkin. This will be very accurate and eliminate any guess work.

Cory281 · Mar 27, 2011

If you want your car to feel faster and pull you back in the seat, get a tuner and CAI. They are around $550, but they make a big difference and is a lot easier to do then gears. American Muscle has some good CAI/programer combos.

Gear Ratio Question

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